1. Field of the Invention
This invention relates generally to the field of process control technology, and, more particularly, it relates to an automated system for controlling the cure cycle for structures or parts made of fiber reinforced composite material and the like, in which the physical characteristics such as temperature and viscosity of the part itself, are used as active process control parameters.
2. Brief Description of the Prior Art
As their name implies, fiber-reinforced composite materials (also known simply as "composites") comprise a base or substrate material, such as an epoxy resin, which is impregnated, for structural strength, with fibers of such materials as carbon, graphite, glass, boron, and nylon. Composites typically exhibit extremely high strength-to-weight ratios, and, accordingly, their use is becoming increasingly popular in the aerospace industry.
One problem still associated with composites is the relatively high cost of fabricating structural parts, such as, for example, aircraft fuselage, wing, and tail sections, from composite materials. A significant factor in the cost of fabricating such parts lies in the care required to control the shape and thickness of the part throughout the fabrication process, while also achieving the necessary degree of structural integrity and strength of the part. Accordingly, new means are constantly being sought for achieving these ends more efficiently and at lower cost.
In fabricating a structure, such as an aircraft part, from composite materials, usually the part is first shaped by laying a preselected number of layers of raw or partially cured composite material on a mold. When the desired shape is achieved, the composite is subjected to a final curing process by placing the part in a pressurized oven known as an autoclave. In the autoclave, polymerization of the resin substrate is completed so that the molded shape is made permanent and the composite material is made hard and durable. Strictly speaking, the final curing process usually completes the final cross-linking of the prepolymerized resin substrate.
Controlling the curing process presents some difficult problems. For example, the viscosity of the resin substrate changes during the curing process and, at times, the viscosity is low enough so that the resin is quite capable of flowing. Controlled flow of the resin is desired at certain times during curing to achieve the required part shape thickness and a structural strength. However, if the resin is allowed to flow in an uncontrolled manner, undesired micro-void formations and/or variations in the thickness of the part can occur. These anomalies can be minimized or controlled by appropriate applications of pressure and temperature during the cure cycle.
For any given part geometry, the flow of the resin is determined by its viscosity and ambient pressure. The viscosity of the resin, in turn, is a function of temperature and of the time the resin has already been subjected to the final curing process. Thus, as temperature and pressure are varied during the curing cycle, the resin will undergo changes in viscosity. For each particular resin, and given temperature variation during the curing cycle, it is possible to empirically determine a "viscosity behavior profile" or "VBP." Given the empirically-derived VBP for a particular resin, it is possible to theoretically predict the viscosity of that resin at any time during the curing cycle. In accordance with the foregoing, present day curing process control techniques monitor temperature and pressure during the cure cycle and manually adjust these parameters in the expectation of achieving a viscosity at any point in time which closely matches the viscosity predicted on the basis of the VBP.
The above noted prior art method of controlling the curing process has shortcomings, however. More particularly, it has been found that the actual viscosity of the resin during the curing cycle often does not parallel closely the expected viscosity behavior profile. This aberration is due to a variety of factors, the most notable ones being variations in the moisture content of the resin and in the production techniques used to make the pre-impregnated resin ("pre-preg"). Furthermore, the final polymerization and cross-linking reactions of the resin which ideally occur only during the final curing step, also occur, albeit at a slow and varying rate, in the pre-impregnated resin. Therefore, even resins having the same original chemical composition may have different states of polymerization when the final curing process is initiated, and therefore will display somewhat different viscosity behavior profiles under the same cure process conditions. Accordingly, a prediction of expected viscosity during a particular point in the cure process based solely upon measurement of temperature and pressure yields results that are, at best, poor approximations of the actual viscosity values.
Part thickness and resin/fiber ratio are functions of the flowing ability of the resin. Furthermore, the flowing ability of the resin is related to its viscosity. Therefore, it can be appreciated that an improved ability to monitor or predict the viscosity of the resin during the curing process is likely to lead to improved ability to finely control part thickness and resin fiber ratios, and therefore the structural strength of the composite parts.
Another factor having an effect on the structural characteristics of the finished composite part is the porosity of the cured composite material. This porosity is a result of the presence of microscopic voids ("microvoids") produced by the release of volatile materials (usually water vapor) during the cure process. It is usually desired to minimize the porosity of the finished part, and this can be achieved through judicious application of pressure during the curing cycle. However, the effectiveness of such application of pressure for the purpose of minimizing the porosity of the composite material is dependent upon the actual viscosity behavior profile during the cure cycle. Moreover, since it is difficult to ascertain accurately the moisture content of the pre-impregnated resin prior to the initiation of the cure cycle, and since the formation of microscopic voids in the materials during the cure cycle cannot be measured or monitored in any meaningful way, the pressure control of porosity during the curing cycle is likely to be non-optimal without means for substantially continuously monitoring the actual viscosity of the resin during the curing cycle.
In view of the foregoing, a two-fold need has been felt in the industry for improved monitoring and control of the curing process. First, it has been a desired goal in the industry to monitor actual temperature, pressure and most importantly, viscosity at various points on the part at various times during the curing cycle, and to control the applied temperature and pressure in accordance with a comparison between a desired temperature, pressure and viscosity profile and the actual, or sensed, profiles. Second, there has been a need for reliably minimizing the development of microvoids in the part as the part is cured.
With regard to the viscosity monitoring function, one method that has been explored in the prior art, measures the viscosity of the substrate through changes in the substrate's dielectric properties. See, for example, Mayberry, "Dielectric Cure Monitoring of Polyimides,1" publishd in In-Process Quality Control for Non-Metallic Materials by U.S. Army Materials and Mechanics Research Center, Watertown, Mass. (1980). While this method has shown some promise, it has yet to see significant commercial application, due, in large part, to a lack of reliability, and unacceptably low signal-to-noise ratios.
With regard to the problem of minimizing the development of microvoids, little has been accomplished to date in the way of "in-process" methods. Rather, emphasis has been placed on non-destructive evaluation of the cured part to ascertain its porosity, and then adjusting the processing parameters based upon this "after-the-fact" evaluation. The most promising non-destructive evaluation technique involves the use of either acoustic emission analysis or acoustic stress wave factor analysis. Both of these techniques make use of the fact that an analysis of the acoustic properties of the material can yield important data relating to its structural characteristics. In the acoustic emission technique, the sample is stressed or loaded in a predetermined way, and the resultant low amplitude, ultrasonic noise is analyzed. See for example Green and Landy, "Acoustic Emission NDE for Advanced Composite Structures," Acoustic Emission Technology Corporation, 1979.
In the acoustic stress wave factor technique, an ultrasonic pulse is transmitted through the material for a given distance and then received. The received pulse is then analyzed, and the results of this analysis yield data related to the structural characteristics of the material. See, for example, Vary and Lark, "Co-relation of Fiber Composite Tensile Strength with the Ultrasonic Stress Wave Factor," Journal of Testing and Evaluation, Volume 7, No. 4, (1979). While in-process use of the acoustic emission technique has been attempted, the need for stressing or loading the material while it is in the autoclave has made the practical application of this method difficult.
Accordingly, the need has been felt in the industry for a method for accurately and non-destructively monitoring the viscosity of the substrate during the curing cycle, and for using the results of the viscosity measurements for interactively controlling autoclave temperature and pressure. A further need has been felt for a practical, in-process mechanism for controlling or minimizing the formation of micorvoids so as to produce composite parts of low porosity.